Science education in the Arab world is conducted in various multilingual contexts. When science is taught in Arabic, diglossia -- the coexistence of the formal language of literacy alongside a local spoken variety -- constitutes a multilingual setting the pedagogical implications of which need to be understood. This study compares teacher--student interaction in two first grade elementary classrooms in Lebanon where science was taught in Arabic by two teachers with different preferences regarding the use of Arabic language varieties. Four lessons in each classroom were video- and audio-recorded and later transcribed. Teacher and student utterances were coded at six levels: class, participant, Arabic language variety, length of utterance, move, and function. Quantitative analyses are reported and qualitative illustrations of key patterns are presented and analysed. Typical IRF patterns of classroom discourse as well as differences in the purposes to which IRF sequences were put could be identified and the roles played by Arabic language varieties described. The findings are discussed from three perspectives: a sociocultural perspective on language in teaching and learning; a three-pronged perspective on multilingual classrooms in which language can be viewed as a problem, a right and a resource; and the sociolinguistics and psychology of Arabic diglossia.